iOS UIImagePNGRepresentation base64编码没有给出预期的输出

The code

代码

static func encodeImage(image: UIImage) -> String{
    var imageData = UIImagePNGRepresentation(image)
    let base64 = imageData.base64EncodedStringWithOptions(NSDataBase64EncodingOptions.allZeros)
    return base64
}
static func encodeImage(image: